Impact-Based Thresholds for Investigation of High-Tide Flooding in the United States

Abstract

High-tide flooding—minor, disruptive coastal inundation—is expected to become more frequent as sea levels rise. However, quantifying just how quickly high-tide flooding rates are changing, and whether some places experience more high-tide flooding than others, is challenging. To quantify trends in high-tide flooding from tide-gauge observations, flood thresholds—elevations above which flooding begins—must be specified. Past studies of high-tide flooding in the United States have used different data sets and approaches for specifying flood thresholds, only some of which directly relate to coastal impacts, which has lead to sometimes conflicting and ambiguous results. Here we present a novel method for quantifying, with uncertainty, high-tide flooding thresholds along the United States coast based on sparsely available impact-based flood thresholds. We use those newly modeled thresholds to make an updated assessment of changes in high-tide flooding across the United States over the past few decades. From 1990–2000 to 2010–2020, high-tide flooding rates almost certainly (probability $P>99\%$) increased along the United States East Coast, Gulf Coast, California, and Pacific Islands, while they very likely $\left(P=93\%\right)$ decreased along Alaska during that time; significant changes in high-tide flooding rates between the two decades were not detected in Oregon, Washington, and the Caribbean. Averaging spatially, we find that high-tide flooding rates probably $\left(P=85\%\right)$ more than doubled nationally between 1990–2000 and 2010–2020. Our approach lays a foundation for future studies to more accurately model high-tide flood thresholds and trends along the global coastline.